Planning with complex actions

In this paper we address the problem of planning with complex actions. We are motivated by the problem of automated Web service composition, in which planning must be performed using predefined complex actions or services as the building blocks of a plan. Planning with complex actions is also compelling in primitive action planning domains because it enables the exploitation of reusable subplans, potentially improving the efficiency of planning. This paper provides a formal, semantically-justified account of how to plan with complex actions using operator-based planning techniques. A key contribution of this work is the definition, characterization, and computation of preconditions and conditional effects for complex actions. While we use the situation calculus and Golog to formalize the task and our solution, the results in this paper are directly applicable to most action theories and planning systems. In particular, we have developed a PDDLequivalent compiler that computes the preconditions and effects of complex actions, thus enabling wide-spread use of these results. Finally we provide an approach to planning that enables us to exploit deductive plan synthesis or alternatively ADL planners to plan with complex actions. Our approach to complex-action planning is sound and complete relative to the corresponding primitive action domain.